what is olfactory indicator 

Hi,

Olfactory indicators are the substances whose smell (or odour) changes in acidic or basic solutions. When an acid or base is added to it, then its characteristic smell cannot be detected.

For e.g. a cloth with onion smell, if treated with sodium hydroxide solution, the smell of onions vanishes. Hydrochloric acid does not change the smell of onions when treated.

Vanilla extract is also an olfactory indicator.